EF - The designation Canon uses for it's standard lens mount. All EF lenses will fit on all EOS bodies.

EF-S - The designation used for lenses which only cover the area of APS-C digital sensors and which can only be mounted on bodies with an EF-S compatible lens mount - currently (04/07) the Digital Rebel, XT, XTi, EOS 20D and EOS 30D.